This invention generally relates to power transmission devices or actuators of the rotary-to-linear motion converting type. Such devices are widely used wherever a rotary power input, such as that furnished by an electric motor or the like, is desired to provide a linear output. This invention more particularly relates to "harmonic screw" actuators of the general type disclosed in my prior U.S. Pat. No. 4,557,153, wherein rotary motion of the power input component is converted into linear motion of the output component by means of a strain wave imposed upon a flexible sleeve member of the assembly. In this prior patent, the flexible sleeve is of a relatively thick wall construction and arranged to threadably engage with a sleeve forming an outer wall of the actuator, which is caused to undergo linear movement incident to rotary motion of the power input component.